Call Setup Example 1

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Call Handling in MSC Server System-

Call Setup Example 1



Call Handling in MSC Server System - Call Setup Example 1


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Table of Contents:

1 UE to MS Call with Fast Forward IP Bearer Establishment ................................ 4 2 Originating RAB ................................................................................................. 5 3 IAM .................................................................................................................... 6 4 APM .................................................................................................................. 7 5 Core Bearer Establishment ................................................................................ 8 6 Terminating TCH ............................................................................................... 9 7 ACM ................................................................................................................ 10 8 Alerting ............................................................................................................ 11 9 Answering ........................................................................................................ 12





1 UE to MS Call with Fast Forward IP Bearer

EstablishmentThis example illustrates the call set-up procedure for a 3G-to-2G call over an IP corenetwork, and using BICC call control signalling.

The example shows fast forward bearer establishment. Note that the delayed forwardbearer establishment alternative is also supported by the MSS.

”Forward” means that the originating side reserves the bearer. ”Fast” means that theoriginating side resources are reserved prior to sending the IAM message. If this isnot possible - for instance if codec negotiation is necessary - delayed forward setupmust be used instead.

The bearer information is sent ”tunnelled” in the BICC IAM message in the forwarddirection and in the APM message in the backward direction. ”Tunnelled” means thatthe BICC protocol does not specify any functions or structures for IP bearerinformation. Instead, it uses IP Bearer Control Protocol (IPBCP) to describe the IPbearer information such as IP addresses, UDP port numbers and codecs used.





2 Originating RAB

The RNC sends a SETUP message using RANAP signalling. The MSS respondswith the RANAP CALL PROCEEDING message.

The MSS requests an access side termination from the MGW and the MGWresponds with bearer details.

Next, the MSS asks for a RAB assignment. The message includes the MGWaddress, the BCU-ID and other bearer parameters.

The AAL2 bearer establishment is requested by the RNC with an ERQ ATMsignalling message. The MGW responds with an ECF message. The access sidebearer is now established.

Upon receipt of the AAL2 ECF message, the RNC indicates to the MSS that the RABassignment has been completed.





3 IAM

In the case of an IP bearer, the MSS always selects the core network side MGW forthe call before sending an IAM towards the succeeding node.

An MGW that has the IP connectivity towards the succeeding node is selected.

The MSS requests the network side bearer reservation from the MGW. With thenetwork side termination reservation a one-way through-connection is establishedbetween the outgoing and incoming termination.

The MGW then sends the bearer-related information (including the IP address andport number) with a Notify Request message - a procedure called Tunnel InformationUp in 3GPP specifications - and the MSS acknowledges the message.

The MSS sends a BICC IAM message to the terminating MSS. Since the FastForward bearer establishment method is used, the IAM contains the tunnelling (i.e.bearer related) information.





4 APM

The terminating MSS sends a BSSAP SETUP message to the BSC after paging hasbeen completed. The BSC then responds with a CALL CONFIRMED message.

The network side termination is requested from the MGW. The Add request messagecontains the bearer-related information received in the IAM. This is called TunnelInformation Down procedure in 3GPP specifications. The MGW reserves the networkside termination and acknowledges with an Add reply message.

Next, the MGW sends its own bearer-related information - including the selected IPaddress and port number - in a Notify request message to the MSS. The MSSresponds with a Notify reply message.

Now a BICC APM message, including the bearer-related information of theterminating MGW, is sent to the originating MSS.





5 Core Bearer Establishment

The MSS sends the bearer-related information received in the APM message to theMGW in a Modify request message.

At this point, both MGWs know each others IP addresses and other bearer-relatedinformation. The bearer over the backbone network is now considered to beestablished.





6 Terminating TCH

Next, the terminating side MSS initiates the reservation of a TDM circuit towards theBSC. Note that this example assumes that the base station subsystem (BSS) sidetermination in the MGW is reserved after the core network bearer has beenestablished. It is beneficial, however, to use early termination reservation instead, sothat the MGW selection can be optimised.

The MSS hunts an outgoing circuit from the MGW and then sends a BSSAP TCHASSIGNMENT REQUEST message to the BSC and the terminating side accessbearer is set up. Upon receipt of the TCH ASSIGNMENT COMPLETE message, aBICC ACM message is sent to the originating MSS.





7 ACM

The originating side MSS, after receiving the ACM message, sends a Modify requestcommand to the MGW, instructing the MGW to make a two-way through-connectionbetween the access side and the network side bearer terminations.

When Stream Mode is used in the MGW, the two-way through-connection betweenthe terminations is reached by modifying the Stream Mode setting of the access sidebearer termination.





8 Alerting

The destination terminal is now ringing and the BSC sends a BSSAP ALERTINGmessage to the MSS.

The MSS requests the MGW to generate a ringing tone with an AddRequestmessage.

The MGW connects the ringing tone.

A BICC CPG message is sent to the originating MSS, which in turn sends a RANAPALERTING message to the RNC.





9 Answering

When the terminal answers, the BSC sends a CONNECT message to the MSS.

The MSS requests the MGW to disconnect the ringing tone and to make a through-connection between the access side and network side bearer terminations. Nowthere is a two-way end-to-end bearer connection.

An ANM message is sent to the originating MSS, which in turn sends a CONNECTmessage to the RNC.

The call is now established.

Call Setup Example 1

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